Steam generators convert heat from the primary side of a nuclear power plant to steam on the secondary side so that the primary and secondary systems are kept separate. A typical generator is a vertical cylinder consisting of a large number of U-shaped tubes which extend from the floor or "tube sheet" of the generator. High temperature and pressure fluid from the reactor travels through the tubes giving up energy to a feed water blanket surrounding the tubes in the generator creating steam and ultimately power when later introduced to turbines.
Steam generators were designed to last upwards of forty years but in practice such reliability figures have proven not to be the case. The problem is that sludge from particulate impurities suspended in the feed water forms on the tubes which greatly affects the efficiency of the generator and can even cause the tubes to degrade to the point of causing fissures in the tubes. If radioactive primary fluid within the tubes seeps into the secondary side, the result can be disastrous. Plugging or otherwise servicing such fissures is time consuming and results in expensive down time during which power must be purchased from other sources at a great expense.
There are known methods for cleaning the tubes proximate the bottom of the steam generator using flexible lances and the like which clean the tubes using water under pressure, but since a typical steam generator can be thirty feet tall, it is difficult to reach the sludge at the upper levels of the tubes using water jets. So, chemical cleaning is used but there are several disadvantages. First, chemical cleaning is very expensive (from $5,000,000 to $10,000,000 per application) and requires an extended outage. Also, some corrosion of steam generator internals by the solvents used will occur during the cleaning. In addition, large quantities of hazardous, possibly radioactive waste may be generated. Disposal of this waste is very expensive. For these reasons, although many utilities have considered chemical cleaning, few plants have actually implemented chemical cleaning.
On the other hand, there are severe technical challenges faced when considering alternate cleaning methods. A typical steam generator has approximately 50,000 square feet of heat transfer area. The tube bundle is about 10 feet in diameter and 30 feet tall but the access alley in the middle of the tube bundle is only 3.5 inches wide and is interrupted by support plates approximately every 4 feet. There are flow slots through the support plates but they are very small in size, typically 2.75 by 15 inches. In addition, the access into the steam generator is limited to a six inch hand hole. Finally, inter tube gaps are only 0.406 wide or smaller.
Thus, the inherent design parameters of a typical steam generator make it difficult to incorporate water jet sludge lancing techniques at the upper tube bundles even though these techniques are adequate to clean the tubes at the level of the tube sheet at the bottom most portion of the steam generator. See, e.g. U.S. Pat. Nos. 4,700,662; 4,980,120; 4,887,555; 4,676,201; and 4,769,085.